A little used feature hidden in the window/utilities menu, Average frames to reduce noise, was written back when I was working on Mystic Knights of Tir Na Nog.
The show was shot on 16mm film. We had to create background plates to composite CG creatures in front of, and the still frames from the shows footage was often very noise.
Therefore, we merged 5-10 frames together to reduce the noise and create a cleaner plate. Hence the feature.
Basically, just load in an avi clip, go to the first frame you want to use, select the utility, and tell it how many frames you want to merge. Before that, I was using "another product" to load in each frame individually into layers, setting their opacity for each layer, and merging them. The whole process tool a while.